Different kinds of means are known for protecting electrically fragile parts of wind turbines from lightning. One kind of means is intended for avoiding the electrical current from a lightning striking the blades of the wind turbine from entering the generator and other electrical and electronic components situated in the top of the wind turbine. This is done by conducting the electrical current from the blades to the tower of the wind turbine and to the ground.
WO 01/86144 describes a wind turbine with such a lightning protection system. The lightning protection system comprises a rod mounted to the blades of the wind turbine. The stick is mounted at the base of the blades. The rod is intended as lightning conducting means conducting the lightning from the base of the blades to a conducting ring mounted co-axially around the main shaft, outside the housing of the nacelle. From this conducting ring the electrical current is conducted through the machine carrying elements provided in the nacelle and further on to the tower along a slip means provided at the yawing gear. Thereby, the conducting of the electrical current is independent of the actual rotary position of the machine carrying elements in the nacelle compared to the tower.
The apparatus described in the above WO-publication is said to be capable of ensuring that no electrical current from the lightning damages the electrical and electronic components. However, the invention is not so described as to enable the person skilled in the art to obtain this object. Also, the means by which the lightning protection is provided has disadvantages. Firstly, providing a separate conducting ring outside and around the entire nacelle demands a very large ring and when mounted, though very difficult, inflicting the aerodynamic properties of the wind turbine nacelle. Also, how the large conducting ring is suspended in relation to the machine carrying elements of the nacelle is not described. Accordingly, the mounting of the conducting ring, which, as mentioned, is one of the major disadvantages of the apparatus, is not mentioned. Therefore, the person skilled in the art will not only be posed to the disadvantages apparent from the description, i.e. the large conducting ring, but will also have to apply a very inventive skill in order to overcome the problem of how to mount the large conducting ring.
DE 197 48 716 describes a lightning protection system comprising at least two conductors inside the turbine blade. The one conductor leads to one side of a transformer, said transformer being capable of generating electrical current in the conductor for heating the blade. However, the one conductor also functions as lightning conductor. In this case, the conductor is connected either directly to the hub as shown in FIG. 1, or to a separate ring provided around the main axle and outside the nacelle of the wind turbine as shown in FIG. 2, or to a separate lightning rod as shown in FIG. 3 and situated outside the nacelle and conducting the lightning voltage to any metal part in the vicinity of the lightning rod.
EP 0 718 495 describes a lightning conductor very like the one shown and described in WO 01/86144. Thus, a separate conductor ring is provided around the main shaft outside the nacelle. A conductor plate, also provided outside the nacelle conducts the lightning form the separate conductor ring to the tower of the wind turbine and therefrom to the ground. The lightning voltage is passed from the turbine blade via a spark gap to the separate conductor ring, from the separate conductor ring via a spark gap to the separate conductor plate and from the separate conductor plate via a spark gap to the tower.
EP 1 154 537 describes a lightning conductor comprising a slip ring provided around the main shaft and brushes passing lightning voltage from the slip ring to a fixed structure secured to the body of the wind turbine. The fixed structure serves as a protection shield for the generator of the wind turbine. The lightning voltage is passed from lightning conductors in the blade along an exteriorly placed cable to the slip ring, and from the slip ring to brushes and the protection shield being part of the fixed structure of the turbine.
It is the object of the present invention to provide a lightning protection means for a wind turbine, said means providing the necessary protection of the electrical and electronic components of the wind turbine, and being easy to install, both in the sense of the craftsman-like skills necessary, and also in sense of the number of and the type of components used, seeing that the lightning protection means is to be installed and serviced in the nacelle of the wind turbine, high above ground level or sea level.